Advances in the synthesis of bioactive unnatural amino acids and peptides

Leucine zipper as a bridge for transaminase self-assembly: A fusion enzyme for efficient chiral conversion of d-phenylglycine

Amino acid transferase is a family of enzymes used to catalyze and separate chiral amino acids. However, due to the low efficiency, by-products and reverse reactions occur in cascade reactions. Therefore, in the research, phenylglycine aminotransferase and aspartate aminotransferase were self-assembled in vitro by leucine zipper. The self-assembled enzyme system with d-phenylglycine and α-ketoglutarate as substrates were used for the chiral transformation reaction. By studying the enzyme combination, kinetic reaction stability and catalytic efficiency, it was found that the self-assembled enzyme showed improved stability and better affinity to the substrate than the control and achieved only ee value of 17.86% for the control at the substrate ratio was 1:2. In contrast, the self-assembled enzyme basically catalyzed the complete conversion of d-Phg to l-Phg, with the ee value as 99%. These results demonstrated the feasibility of the leucine zipper and the conversion of d-phenylglycine to the l-type by fusion enzyme.

Insights into Recent Nickel-Catalyzed Reductive and Redox C-C Coupling of Electrophiles, C(sp3)-H Bonds and Alkenes

ConspectusTransition metal-catalyzed reductive cross-coupling of two carbon electrophiles, also known as cross-electrophile coupling (XEC), has transformed the landscape of C-C coupling chemistry. Nickel catalysts, in particular, have demonstrated exceptional performance in facilitating XEC reactions, allowing for diverse elegant transformations by employing various electrophiles to forge C-C bonds. Nevertheless, several crucial challenges remain to be addressed. First, the intrinsic chemoselectivity between two structurally similar electrophiles in Ni-catalyzed C(sp3)-C(sp3) and C(sp2)-C(sp2) cross-coupling has not been well understood; this necessitates an excess of one of the coupling partners to achieve synthetically useful outcomes. Second, the substitution of economically and environmentally benign nonmetal reductants for Zn/Mn can help scale up XEC reactions and avoid trace metals in pharmaceutical products, but research in this direction has progressed slowly. Finally, it is highly warranted to leverage mechanistic insights from Ni-catalyzed XEC to develop innovative thermoredox coupling protocols, specifically designed to tackle challenges associated with difficult substrates such as C(sp3)-H bonds and unactivated alkenes.In this Account, we address the aforementioned issues by reviewing our recent work on the reductive coupling of C-X and C-O electrophiles, the thermoredox strategy for coupling associated with C(sp3)-H bonds and unactivated alkenes, and the use of diboron esters as nonmetal reductants to achieve reductive coupling. We focus on the mechanistic perspectives of the transformations, particularly how the key C-NiIII-C intermediates are generated, in order to explain the chemoselective and regioselective coupling results. The Account consists of four sections. First, we discuss the Zn/Mn-mediated chemoselective C(sp2)-C(sp2) and C(sp3)-C(sp3) bond formations based on the coupling of selected alkyl/aryl, allyl/benzyl, and other electrophiles. Second, we describe the use of diboron esters as versatile reductants to achieve C(sp3)-C(sp3) and C(sp3)-C(sp2) couplings, with an emphasis on the mechanistic consideration for the construction of C(sp3)-C(sp2) bonds. Third, we discuss leveraging C(sp3)-O bonds for effective C(sp3)-C bond formation via in situ halogenation of alcohols as well as the reductive preparation of α-vinylated and -arylated unusual amino esters. In the final section, we illustrate the thermoredox functionalization of challenging C(sp3)-H bonds with aryl and alkyl halides to afford C(sp3)-C bonds by taking advantage of the compatibility of Zn with the oxidant di-tert-butylperoxide (DTBP). Furthermore, we discuss a Ni-catalyzed and SiH/DTBP-mediated hydrodimerization of terminal alkenes to selectively forge head-to-head and methyl branched C(sp3)-C(sp3) bonds. This process, conducted in the presence or absence of catalytic CuBr2, provides a solution to a long-standing challenge: site-selective hydrocoupling of unactivated alkenes to produce challenging C(sp3)-C(sp3) bonds.

Acid-Catalyzed Oxy-aminomethylation of Styrenes

We report a strong Brønsted acid-catalyzed three-component oxy-aminomethylation of styrenes with sym-trioxane and sulfonamides or carbamates. This transformation provides a variety of 1,3-oxazinanes in moderate to good yields under mild reaction conditions. The obtained heterocycles can be readily transformed into the corresponding 1,3-amino alcohols, which are useful building blocks for the synthesis of pharmaceutically relevant molecules. Mechanistic studies suggest the intermediacy of an in situ formed 1,3,5-dioxazinane and a subsequent reaction with the olefin.

Photoredox Metal-Free Synthesis of Unnatural β-Silyl-α-Amino Acids via Hydrosilylation

An efficient, practical and metal-free methodology for the synthesis of β-silyl-α-amino acid motifs via photoredox and hydrogen atom transfer (HAT) process is described. This protocol enables the direct hydrosilylation of dehydroalanine derivatives and tolerates a wide array of functional groups and synthetic handles, leading to valuable β-silyl-α-amino acids with moderate to good yields.

Lubricin-Inspired Loop Zwitterionic Peptide for Fabrication of Superior Antifouling Surfaces

Biofouling represents great challenges in many applications, and zwitterionic peptides have been a promising candidate due to their biocompatibility and excellent antifouling performance. Inspired by lubricin, we designed a loop-like zwitterionic peptide and investigated the effect of conformation (linear or loop) on the antifouling properties using a combination of surface plasma resonance (SPR), surface force apparatus (SFA), and all atomistic molecular dynamics (MD) simulation techniques. Our results demonstrate that the loop-like zwitterionic peptides perform better in resisting the adsorption of proteins and bacteria. SFA measurements show that the loop-like peptides reduce the adhesion between the modified surface and the modeling foulant lysozyme. All atomistic MD simulations reveal that the loop-like zwitterionic peptides are more rigid than the linear-like zwitterionic peptides and avoid the penetration of the terminus into the foulants, which lower the interaction between the zwitterionic peptides and foulants. Besides, the loop-like zwitterionic peptides avoid the aggregation of the chains and bind more water, improving the hydrophilicity and antifouling performance. Altogether, this study provides a more comprehensive understanding of the conformation effect of zwitterionic peptides on their antifouling properties, which may contribute to designing novel antifouling materials in various biomedical applications.

Synthesis of β-Hydroxy α-Amino Acids Through Brønsted Base-Catalyzed syn-Selective Direct Aldol Reaction of Schiff Bases of Glycine o-Nitroanilide

Here we report the highly enantio- and syn-selective synthesis of β-hydroxy α-amino acids from glycine imine derivatives under Brønsted base (BB) catalysis. The key of this approach is the use of benzophenone-derived imine of glycine o-nitroanilide as a pronucleophile, where the o-nitroanilide framework provides an efficient hydrogen-bonding platform that accounts for nucleophile reactivity and diastereoselectivity.

Radical-Based Synthesis and Modification of Amino Acids

Amino acids (AAs) are key structural motifs with widespread applications in organic synthesis, biochemistry, and material sciences. Recently, with the development of milder and more versatile radical-based procedures, the use of strategies relying on radical chemistry for the synthesis and modification of AAs has gained increased attention, as they allow rapid access to libraries of novel unnatural AAs containing a wide range of structural motifs. In this Minireview, we provide a broad overview of the advancements made in this field during the last decade, focusing on methods for the de novo synthesis of α-, β-, and γ-AAs, as well as for the selective derivatisation of canonical and non-canonical α-AAs.

Nα-Amino acid containing privileged structures: design, synthesis and use in solid-phase peptide synthesis

Fmoc-protected Nα-amino acid containing heterocyclic privileged structures, O-(1-methyl-5-oxo-2,5-dihydro-1H-pyrrol-3-yl)-l-serine and O-((S)-5-oxo-2,3,5,7a-tetrahydro-1H-pyrrolizin-7-yl)-l-serine, were synthesized on the solid phase from simple commercially available building blocks under mild conditions. The amino acid side-chain is composed of tetramic acid, a natural product derived privileged structure. The key transformation was the formation of cyclic enol ethers via nonclassical Wittig olefinations of the esters. Solid-phase synthesis represents a method of choice, particularly for the synthesis of peptides. This route is compatible with traditional Merrifield solid-phase peptide synthesis (SPPS), as documented on the preparation of the pentapeptide Leu-enkephalin amide H-Tyr-Gly-Gly-Phe-Leu-NH2 with Phe or Tyr replaced by a novel amino acid.

Natural and synthetic peptides with antifungal activity

In recent years, the increase of invasive fungal infections and the emergence of antifungal resistance stressed the need for new antifungal drugs. Peptides have shown to be good candidates for the development of alternative antimicrobial agents through high-throughput screening, and subsequent optimization according to a rational approach. This review presents a brief overview on antifungal natural peptides of different sources (animals, plants, micro-organisms), peptide fragments derived by proteolytic cleavage of precursor physiological proteins (cryptides), synthetic unnatural peptides and peptide derivatives. Antifungal peptides are schematically reported based on their structure, antifungal spectrum and reported effects. Natural or synthetic peptides and their modified derivatives may represent the basis for new compounds active against fungal infections.

Synthesis of new unnatural N(α)-Fmoc pyrimidin-4-one amino acids: use of the p-benzyloxybenzyloxy group as a pyrimidinone masking group

The p-benzyloxybenzyloxy group is used to mask the oxo function of the 4(3H)-pyrimidinone ring in the synthesis of new unnatural amino acids. The synthetic approach is based on an aromatic nucleophilic substitution reaction between 4-[4-(benzyloxy)benzyloxy]-2-(benzylsulfonyl)pyrimidine and the nucleophilic side chain of several N(α)-Boc amino esters, as the key step, followed by a series of standard protecting group transformations. p-Benzyloxybenzyloxy is efficiently removed under mild acid conditions to recover the 4(3H)-pyrimidinone system.

Experimental challenges of sense codon reassignment: an innovative approach to genetic code expansion

The addition of new and versatile chemical and biological properties to proteins pursued through incorporation of non-canonical amino acids is at present primarily achieved by stop codon suppression. However, it is critical to find new "blank" codons to increase the variety and efficiency of such insertions, thereby taking 'sense codon recoding' to center stage in the field of genetic code expansion. Current thought optimistically suggests the use of the pyrrolysine system coupled with re-synthesis of genomic information towards achieving sense codon reassignment. Upon review of the serious experimental challenges reported in recent studies, we propose that success in this area will depend on the re-synthesis of genomes, but also on 'rewiring' the mechanism of protein synthesis and of its quality control.

Screening nonspecific interactions of peptides without background interference

The need to discover new peptide sequences to perform particular tasks has lead to a variety of peptide screening methods: phage display, yeast display, bacterial display and resin display. These are effective screening methods because the role of background binding is often insignificant. In the field of nonfouling materials, however, a premium is placed on chemistries that have extremely low levels of nonspecific binding. Due to the presence of background binding, it is not possible to use traditional peptide screening methods to select for nonfouling chemistries. Here, we developed a peptide screening method, as compared to traditional methods, that can successfully evaluate the effectiveness of nonfouling peptide sequences. We have tested the effect of different peptide lengths and chemistries on the adsorption of protein. The order of residues within a single sequence was also adjusted to determine the effect of charge segregation on protein adsorption.